This invention relates to the processing of digital N.T.S.C. colour television signals.
Our British Pat. Nos. 1,534,268-270 have described the use with digital PAL colour television signals of separated signals in the form of a luminance signal Y.sub.2 sampled at twice the colour subcarrier frequency and a composite chrominance signal C.sub.1 sampled at the colour subcarrier frequency f.sub.sc itself. The total sample rate requirement is thus three times the colour subcarrier frequency. The composite chrominance signal was in a special form, consisting of the sum and difference of the colour difference signals U and V on alternate lines, i.e. U+V on one line and U-V on the next.
The Y.sub.2 /C.sub.1 combination is in a form which can be convenient for switching and mixing, for transmission, or for recording.
This invention is concerned to provide a solution to problems arising in attempting to derive a corresponding Y.sub.2 /C.sub.1 system for N.T.S.C. signals.
It will be appreciated that the sampling frequency 2f.sub.sc used for the luminance signal is a sub-Nyquist frequency, that is, it is less than twice the maximum video frequency which is to be transmitted. This sub-Nyquist frequency can only be used because of the line periodicity of the television signal. If it is assumed for the moment that there is no line-to-line variation in the signals, the spectral components of the luminance signal will tend to congregate around integral multiples of the line frequency f.sub.L. These spectral components of say nf.sub.L will after sampling cause alias components due to spectrum folding of frequency 2f.sub.sc -nf.sub.L. Now, in the PAL system, f.sub.sc is substantially equal to an odd integral multiple of one-quarter of the line frequency, i.e. (m.+-.1/4)f.sub.L. Thus twice the subcarrier frequency is substantially equal to an odd integral multiple of one-half of the line frequency, i.e. (m'+1/2)f.sub.L. Thus the alias components which are of the form {(m'+1/2(f.sub.L -nf.sub.L } will also always congregate around odd integral multiples of one-half of the line frequency. Using this property it is possible to remove these alias components by means of a comb filter which passes components equal to integral multiples of the line frequency but which stops components which are equal to odd integral multiples of half the line frequency.
Where line-to-line variations occur there will be residual alias components, but in practical television signals the sampling system works well without introducing subjectively noticeable degradation.
However such a filtering system cannot be used with N.T.S.C. signals, because in the N.T.S.C. system the colour subcarrier frequency is itself equal to an odd integral multiple of one-half (rather than one-quarter) of the line frequency F.sub.L, namely (n+1/2f.sub.L). Twice this frequency is an exact multiple of the line frequency. Thus, input signal components which are themselves multiples of the line frequency will also produce alias components which are multiples of the line frequency. These components cannot be separated by comb filtering.
From this it would seem that it is impossible to sample the separated luminance and colour difference baseband signal components at twice the N.T.S.C. colour subcarrier frequency without introducing substantial impairment into the signals.
It should be noted that the sampling of the composite N.T.S.C. signal has already been proposed by John P. Rossi, see SMPTE Journal, Vol. 85, No. 1, Jan. 1976, pages 1 to 6, "Sub-Nyquist-encoded PCM NTSC Colour Television" and IEE Conference Publication No. 166, pages 218 to 221, "Sub-Nyquist sampled PCM NTSC Color TV Signal Derived from Four Times the Color Subcarrier", (International Broadcasting Convention 1978).
An improved method is described in our British Pat. application No. 7902912 (V. G. Devereux), publication No. 2,040,640. The problems involved in encoding to give samples directly representative of an encoded composite N.T.S.C. signal are however quite different from those involved in obtaining the sampled luminance signal Y.sub.2.